1. Field of the Invention
The present invention relates to a method for fabricating capacitors for semiconductor devices and, more particularly, to a method for fabricating capacitors that exhibit both the improved electrical characteristics and the high capacitance values required for advanced semiconductor devices.
2. Description of the Related Art
As is well known, recent developments in semiconductor processing techniques have allowed the successful production of semiconductor products having increasingly high levels of integration. As a result, active research and development efforts continue to be directed toward both reducing cell area and reducing the device operating voltage.
Although high levels of device integration greatly reduce the wafer area available for capacitor formation, the charge capacity preferred for operation of a memory device remains on the order of 25 fF or more per cell despite the reduction in cell area. This level of charge capacity is useful in preventing the generation of soft errors and reducing the refresh time.
Conventional DRAM capacitors commonly use a dielectric film having a stacked nitride/oxide (NO) structure and a three-dimensional lower electrode structure to increase the effective electrode area to obtain sufficient capacitance values.
However, the processing complexities and difficulties inherent in forming such three-dimensional lower electrode structures limits the ability of such processes to provide sufficient capacitance in a consistent and predictable manner. In those instances in which the height of the lower electrode is increased, the depth of focus and dimension control that can be obtained during subsequent photolithography processes may be insufficient to accurately reproduce the necessary patterns. This is because the increased height of the lower electrode produces severe topology differences between the cell and peripheral circuit regions. These topology differences have an adverse effect on subsequent integration processes conducted after a wiring process.
Thus, capacitors utilizing the conventional NO structure are frequently limited in their ability to provide the levels of capacitance required to support the next generation memories having 256 M or more cells.
In order to overcome the limitations associated with conventional NO capacitors and the dielectric constant of 4 to 5 provided by the NO film, a Ta2O5 thin film capable of providing a dielectric constant of 25 to 27 has been proposed as an alternate dielectric film. The higher dielectric constant of the Ta2O5 film permits the construction of capacitors having substantially higher capacitance than similarly sized NO capacitors.
In a nominal Ta2O5 thin film, however, substitutional Ta atoms inevitably exist as a result of composition ratio variations between Ta and O atoms within the film. The nominal stoichiometry, although convenient, does not reflect the inherent chemical instability of the Ta2O5 thin film. In other words, substitutive Ta atoms in the form of oxygen vacancies are always present in the Ta2O5 thin film due to the variable and unstable stoichiometry of the Ta2O5 material.
Furthermore, although the number of oxygen vacancies may be varied somewhat depending on the actual composition and bonding degrees of the incorporated elements, the oxygen vacancies cannot be completely removed from the dielectric thin film.
In order to prevent generation of leakage current in the Ta2O5 capacitors, therefore, it is necessary to perform a separate oxidation process to stabilize the stoichiometry of the Ta2O5 thin film by oxidizing the substitutive Ta atoms present in the dielectric thin film.
Moreover, the Ta2O5 thin film may act as a strong oxidizer when in contact with polysilicon (oxide-based electrodes) or TiN (metal-based electrodes), two materials commonly used in forming the upper and lower electrodes of the capacitors. As a result, oxygen present in the Ta2O5 thin film may migrate to the interfaces between the dielectric layer and the electrodes and react with the electrode materials, thereby forming low-dielectric oxide layers and degrading the uniformity and electrical properties of those interfaces.
During the formation of the Ta2O5 thin film, the organic portions from the Ta(OC2H5)5, a precursor compound used in forming the Ta2O5 film, can react with O2 or N2O gas during the LPCVD process to form various impurities including carbon (C), carbon compounds (such as CH4 and C2H4), and water vapor (H2O), that can be incorporated into the Ta2O5 thin film. As a result of these impurities, as well as other ions, free radicals, and oxygen vacancies present in the Ta2O5 film, the resulting capacitors tend to exhibit increased leakage current and degraded dielectric characteristics.
The method according to the present invention has been developed to overcome the above mentioned problems and limitations experienced and/or inherent in prior art processes and materials. It is an object of the invention to provide a method for fabricating capacitors for semiconductor devices that exhibit improved electrical characteristics while ensuring sufficient capacitance to support next generation semiconductor devices.
Another object of the invention is to provide a method for fabricating capacitors for semiconductor devices that renders unnecessary certain process steps designed to increase the effective capacitor area and thus ensure a sufficiently high capacitance. By allowing these steps to be eliminated, the present invention simplifies the manufacturing process by reducing the number of processing steps, thereby also reducing both the processing time and the associated manufacturing costs.
Another object of the present invention is to provide a method for fabricating capacitors for semiconductor devices that render unnecessary certain thermal and oxidation processes prior to the formation of a dielectric film. By allowing these steps to be eliminated, the present invention further reduces the manufacturing costs and improves productivity.
In one embodiment, the present invention provides a method for fabricating capacitors for semiconductor devices comprising the steps of: forming a lower electrode on a semiconductor substrate that incorporates a variety of previously formed and diverse structures necessary for proper functioning of the final semiconductor device; depositing an amorphous TaON thin film over the lower electrode, and subjecting the deposited amorphous TaON thin film to a low-temperature thermal process and a high-temperature thermal process, thereby forming a TaON dielectric film; and forming an upper electrode on the TaON dielectric film.
In another embodiment, the present invention provides a method for fabricating a capacitor of a semiconductor device, comprising the steps of: forming a lower electrode on a semiconductor substrate formed with diverse structures required for a fabrication of a semiconductor device; nitriding a surface of the lower electrode to form a nitride film on the lower electrode, forming an amorphous TaON thin film over the nitride film and subjecting the amorphous TaON thin film to a low-temperature thermal process at a temperature of 300 to 500xc2x0 C.; then subjecting the TaON thin film to a high-temperature thermal process at a temperature of 650 to 950xc2x0 C., thereby forming a TaON dielectric film; and forming an upper electrode on the TaON dielectric film.
In another embodiment, the present invention provides a method for fabricating capacitors for semiconductor devices comprising the steps of: forming a lower electrode on a semiconductor substrate that incorporates a variety of previously formed and diverse structures necessary for proper functioning of the final semiconductor device; forming a nitride film adapted to nitrify an upper surface of the lower electrode; forming an amorphous TaON thin film over the lower electrode; subjecting the amorphous TaON thin film to a low-temperature thermal process at a temperature of 300 to 500xc2x0 C.; subjecting the TaON film to a high-temperature thermal process at a temperature of 650 to 950xc2x0 C., thereby forming a TaON dielectric film; and forming an upper electrode on the TaON dielectric film.
The above objects, as well as other features and advantages of the present invention will become more apparent in light of the following detailed description and the accompanying figures.